Carrier wave system compensated automatic gain control



CARRIER WAVE' SYSTEM COMPENSATED AUTOMATIC GAIN CONTROL Oct. 4, 1938.

I/YVE/YTOQ Ham/a; E EJ350151 ATTORNEY 60 40 Input Ravage z'iz Deaz'edsPatented Oct. 4, 1938 UNITED STATES CARRIER WAVE SYSTEM- COMP ENSATEDAUTOMATIUGAIN CONTROL 2 Harold F. Elliott, Palo Alto, Calif.

. Application December 19, 1933, Serial No. 703,054

6 Claims.

My invention relates to carrier wave systems such as are used forcommunication by telegraphy, telephony, facsimile and television overwire and radio circuits and especially to systems embodying means forautomatically controlling the output level at the receiving terminals.

One object of my invention is to extend the range of control of suchsystems. Another ob ject is to improve the precision of control. An-

ll) other object is to improve the stability of such systems. Stillanother object is to improve the quality of reproduction. These andother objectives will be described more fully in'the followingdescription and appended claims.

Referring to the accompanying drawing in which like numerals ofreference indicate similar parts throughout the several views:

Figure 1 is a schematic diagramilustrating one form of my invention;

Figure 2- is a circuit diagram of a carrier wave receiving systemembodying my invention; and

Figure 3 gives curves showing the results obtained by my invention ascompared with previous systems.

In the schematic diagram of a carrier. wave receiver embodying myinvention, as shown in Fig. 1, the incoming carrier waves from wires orradio antenna are applied to the input terminals I, of a carrierfrequency amplifier of one or more 30. stages. By Way of illustration,three carrier frequency amplifiers 2, 3 and 4 are shown, but theinvention is not in any sense restricted to this number of amplifiers asmore or less may be used. The amplified output, appearing at 5, is

applied in suitable proportion to ,a governor 6 which serves to rectifythe carrier waves, with or without further amplification and selection,and to deliver governing potentials at I. A suitable proportion of thesepotentials is utilized through connections 8 for automatically governingor controlling the gain of, amplifiers 2, 3 and. 4.

This much of the system so far described is similar to systems embodyingautomatic gain or volume control now in use. The relation between inputand output of such systems, when well designed, will be about as shownby curve B in Figure 3. A variation of 60 to 80 decibels in the level ofthe signals applied to the input terminals I, is held to a variation of10.to 20 decibels at the output terminals. 5. If it is at tempted tohold the output. variation Within closer limits, the system becomesunstable, Stable governing action can only be'attained when there is amaterial slope, in the input-output curve. While such a variation inoutput is,not

objectionable for some purposes, there are other uses where it isdesirable to have a much more nearly constant output, and one of themain objectives of my invention is the provision of means whichaccomplish this and also extend the'range of control while maintainingstability and high quality of reproduction.

In the form of my invention shown in Figure 1, the desideratumabove-mentioned is carried out by a carrier frequency compensator 9,which 10' receives amplified and governed carrier frequency signals from5 and governing potentials from 1, each in suitable proportion, throughconnections l0 and II. (Compensator 9 by preference takes the form of acarrier frequency amplifier of one it or more stages having provisionsfor a variation of gain which preferably just compensates for thevariations in the level of the governed signals at 5. The output ofcompensator 9 at ter-' minals I2 is then constant, regardless of varia-20- tions at terminals I or 5. The relation of the input at l to theoutput of the system as a whole at I2 is then as shown in curve A,Figure 3. The ouput from terminals I2 may be passed to a furtheramplifier or to a line, or an antenna, or 35 to a demodulator, orutilized in any desired manner. The amplifiers 2, 3 and 4 andcompensator 9 should be of the type in which it is possible to vary thegain over a considerable range without introducing objectionabledistortion. One way. of accomplishing this is to use so-called multi-muelectronic amplifier tubes which have a control characteristic free fromabrupt changes in curvature and which can be used as controlled carrierfrequency amplifiers over wide ranges as without introducingobjectionable distortion.

Before discussing the detailed circuits, it may be well to point outmore specifically why the arrangements shown in Figure 1 make it,possible to obtain precise control over a wide range, an as illustratedby curve A in Figure 3, whereas previous methods have failed either togive satisfactory range or precise control or have introduced seriousdistortion or have been. unstable or had other defects. In the firstplace, (at the circuit arrangement of my invention makes it possible toproportion the carrier frequency amplifier and governor so thatstability, con stancy, freedom from distortion and dependableperformance are attained. Secondly the use vof 50' a compensator,operating at carrier frequency, after the main carrier frequencyamplifier and governor, makes it possible .tocompensate for and correctthe variations in the outputof the amplifier and governor withoutintroducing ob- 5 jectionable distortion, or disturbing the stability orconstancy of operation of the system. By placing acompensator at thispoint in the system, and by suitably proportioning it and the po-'tentials applied to it, it is possible to maintain very constant outputfrom the system as a whole with high quality of reproduction, stability,and dependability. Normal variations in tube constants or potentialshave little effect upon the operation. There 'are no criticaladjustments and any desired operating characteristics within reason areattainable. Previous methods, on

the other hand, have failed to accomplish constant output by a widemargin or 'have been 7 unstable, or have introduced serious distortionor have been critical of adjustment or have Varied in performance withtube and potential changes. t

Figure 2 shows detailed circuits of a carrier frequency receiver, suchas may be used for radio or wire line reception, embodying my invention.Carrier wave signals applied to the input terminals I3 are amplified inthe carrier frequency amplifier tubes l4, I and I6, which are preferablyof the so-called multi-mu or other type permitting a wide range ofamplification control withoutintroducing objectionable distortion. Theamplifier I4I5,I6, may be of the type in which the entire amplificationfunction is carried out'at a singlecarrier frequency, or it may be ofthe type in which the frequency is converted to a lower or higherperiodicity, as is done, forexample, in superheterodyne systemsv If thelatter is used, tube I5 may, if desired, be a frequency converter tubeand a suitableheterodyning frequency may be applied in any convenientmanner, as, for example, through terminals I'I The transformersbetweenstages and the connections for applying andcontrolling' thevarious operating potentials may be of any convenient and suitableform., These are not described in detail here since these matters ,arewell understood, and this description may be simplified by confining itto the details that are new and novel.

In Figure 2,,Irhave shown the output of the last main carrier frequencyamplifier stage I6, as feeding into an output transformer with tunedprimary and secondary, I 8 and I9 but it will be understood thatanyother suitable output termination 'may be employed. A connection 26'leads from 'a suitable point on secondary I9 to the input of governor22. Another connection 2| leads from another suitable point on secondaryI9 to the input of compensator 24. It will be noted that both governor22 and compensator 24 are supplied with oarrier frequency signals fromthe output of amplifier I6- Other arrangements may be employed incarrying out the invention but the, particular arrangement shown hasbeen found very effective and stable in operation as well as relativelysimple in construction.

The governor comprises a carrier frequency amplifier tube 22, and a fullwave rectifier 25, which serves to rectify the output of tube 22 andsupply direct current governing potentials at potentiometer 26. Anyother arrangement supplying suitable governing potentials maybe'employed, but again, the circuits shown have been found to:be bothsimple and effective in practice. Suitable potentials from potentiometer26 may be appllied through a suitable system of filters andconnectionsTLto the gain control circuits of amplifiers I4, I5 and I6.Amplifiers I4, I5 and I6 together with governor 22-25 constitute aself-governing amplifier whose output at secondary I9 is held within thelimits usually obtainable in amplifiers with well designed automaticgain control, as illustrated by curve B in Figure 3.

A suitable proportion of the controlled output from amplifiers I4I5I6 atsecondary I9 may be applied through suitable connections 2| to the inputof compensator 24. Also a suitable propor tion of the governingpotentials at 26 maybe applied through appropriate filters andconnections, as 23, to a gain control electrode or .othersuitable gaincontrol means, in compensator 24. Compensator 24 should be a variable muelectronic broadcast receiver, by employing, standard multimu tubes atI4, I5, I6, 22 and-24, and a standard full wave rectifier at 25. Inthisparticular re.- ceiver, the full rectified potential from 25 was appliedto the gain control circuit 21, of; tubes, I4, I5 and I6 and aboutonehalf,of this potential was appliedrto the gain control circuit 23 ofcome pensator tube 24. Theinitial bias on tubes I4 and I6 was setbyrrheostat 28 and rheostat 29 was .used for the same purpose oncompensator tube 24. It was found thatthe output of com pensator .24,could be'made very constant overa.

very wide range of inputs atfI3 by suitably adjusting the initial biasat 29 and. properly pro-' portioning the. controhpotential taken frompotentiometer 26.

iWhile I have shown compensator:24 asicomprising a single variable mutube, it will be understood that additional tubes or even'entirelydifferent means may be utilized if needed or desired. As :alreadyoutlined, theoutput of the compensator 24 may be passed-to a furtheramplifier, or to a'wire line, or to a demodulator or utilized in anydesired manner. In the broad-i cast receiver shown in Figure 2,compensator 24 is shown as-provided with an output transformer havingtuned primary and. secondary feeding into a full wave'diode rectifier30. The output of the latter is shown feeding into. an audio frequencyamplifier 32 througha potentiometer- 3|, with the output of the systemappearing as audio fre-- quency signals at terminals33. Such anarrangement of a radio receiver is very advantageous since the-rectifier30 or other detector operates at a very constant level which can beproportioned and adjusted for the'highest quality of reproduction andmost effective operation. I 3

Since the audio output of the system is substan: tially constant, some"visual means of indicating when the circuits are tuned to resonance withthe incoming carrier is desirable. Such an indicator is shown at 34 inFigure 2 Indicator 34 may; comprise any desired or knowninstrumentality'. It will be apparent that'many variations may be madewithout" departing from the spirit of P the invention, as defined in theappended claims andthat details of wiring, filtering, tuningp'ro-Qvisions, etc., maybe carried out in an almost'infinite variety ofWays. 3The question may arise asto whethenth e method may be applied to thecompensation of signals at audio frequency and this may be accomplishedif the design of the compensator and the proportion of the signals andpotentials applied. to it are such that the necessary range of controlcan be accomplished without introducing objectionable distortion. Thenecessary control range and other essential features are readilyobtained when the compensator is operated at carrier frequencies asshown in the accompanying diagrams and this is therefore given as thepreferred form of the invention.

With reference to the location of manual means for controlling theabsolute output of a system of the type described, such control meansshould preferably, though not necessarily, follow the compensator 9 inFigure 1 or 24 in Figure 2. If the compensator is followed by a detectorand audio amplifier, the manual control may be on either side of thedetector, or may be in the audio amplifier. If the compensator feeds toa line, the control may be either side of the compensator and may takeany suitable form. The preferred location is after the compensator orafter the detector.

Having thus described my invention, I claim:

1. A superheterodyne radio receiver comprising a radio frequencyamplifier with tuned circuits and a tube having gain control means, afrequency converter having tuned circuits and a tube having gain controlmeans, an intermediate frequency amplifier having tuned circuits and atube having gain control means, a governor comprising an amplifier and adiode rectifier interconnected by a tuned circuit, a circuit connectingthe input of the governor and the output of the intermediate frequencyamplifier, a circuit connecting the output of the rectifier and the gaincontrol means in the radio frequency amplifier, frequency converter andintermediate frequency amplifier, an intermediate frequency compensatorhaving input and output circuits and gain control means wherein the gaincharacteristic inversely and proportionately matches and offsets theinput-output characteristic of the radio frequency amplifier-frequencyconverter-intermediate frequency amplifier and governor combination, acircuit connecting the input circuit of the compensator and the outputof the intermediate frequency amplifier, a circuit with potentialproportioning means connecting the gain control means of the compensatorand the output of the rectifier, a detector with input and outputcircuits, a circuit connecting the input of the detector with the outputof the compensator, an audio amplifier with input and output circuits, acircuit connecting the input to the audio amplifier and the output ofthe detector and a volume control regulating the input to the audioamplifier.

2. A radio receiver comprising a carrier frequency amplifier having aplurality of multi-mu amplifier tubes, input and output circuits andtuning means, a governor comprising an amplifier followed by a dioderectifier, input and output circuits and tuning means, a resistor withtapping means in the output of the rectifier, a connection between theoutput of the carrier frequency amplifier and the input of the governor,connections between the resistor and gain control electrodes in themulti-mu tubes, a compensator comprising a tube having a variable gaincharacteristic which is the complement and inversely proportionate tothe input-output characteristic of the carrier frequencyamplifiergovernor combination, input, output and gain control circuitsfor the compensator, a connection from the compensator input to theoutput of the carrier frequency amplifier, a connection from thecompensator gain control circuits to the resistor, a detector with inputand output circuits, a connection from the detector input to thecompensator output, an audio amplifier with input and output circuits,and a circuit including volume control means interconnecting the outputof the detector and the input of the audio amplifier.

3. In combination, a main carrier frequency amplifier with input andoutput circuits and gain control means, a governor comprising a carrierfrequency amplifier followed by a diode fier, connections from therectifier output to the gain control means, a carrier frequencycompensator comprising input and output circuits and a variable gainelectronic tube having in combination with the potentials appliedthereto an input-output characteristic whose slope is inverselyproportionate and complementary to the slope of the input-outputcharacteristic of the main carrier frequency amplifier and governor,connections from the compensator input to the main carrier frequencyamplifier output, connections from the rectifier output to gain controlmeans in the compensator, a constant level detector connected to theoutput of the compensator and an audio frequency amplifier connectedthrough a volume control to the output of the detector.

4. A superheterodyne receiver in accordance with claim 1 with theaddition of potentially operated tuning indicating means.

5. A radio receiver in accordance with claim 2 with the addition ofpotentially operated tuning indicating means.

6. The combination as set forth in claim 3 with the addition ofresonance indicating means.

HAROLD F. ELLIOTT.

